Diffusion of wireless communication devices including mobile phones has encouraged development of filters formed by combining acoustic wave devices using a surface acoustic wave (SAW) or bulk acoustic wave (BAW). The filter using a SAW or BAW has a small outer shape and a high Q compared to a dielectric filter, and thus is suitable for a high-frequency component in a wireless communication device such as a mobile phone required to be small in size and have a steep skirt characteristic. Moreover, there has been suggested an acoustic wave device using a Lamb wave as a developed device of the acoustic wave device using a SAW or BAW.
In recent years, filters are required to have high performance. For example, a bandwidth of a filter characteristic is required to be widened. The bandwidth of the filter characteristic can be widened by increasing an electromechanical coupling coefficient of an acoustic wave device used in the filter. Use of a piezoelectric film with a high electromechanical coupling coefficient can increase the electromechanical coupling coefficient of the acoustic wave device.
When an aluminum nitride film is used as the piezoelectric film, the electromechanical coupling coefficient of the acoustic wave device can be improved by controlling a c-axis orientation of the aluminum nitride film as disclosed in Rajan S. Naik, and 10 others, “Measurements of the Bulk, C-Axis Electromechanical Coupling Constant as a Function of AlN Film Quality”, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS AND FREQUENCY CONTROL, 2000, vol. 47, p. 292-296 (Non-Patent Document 1), for example. For example, the electromechanical coupling coefficient of the acoustic wave device can be improved by using an aluminum nitride film containing an alkali earth metal and/or a rare-earth metal for the piezoelectric film as disclosed in Japanese Patent Application Publication No. 2002-344279 (Patent Document 1). Moreover, piezoelectric response of the acoustic wave device can be improved by using an aluminum nitride film containing scandium at a content rate in a predetermined range for the piezoelectric film as disclosed in Japanese Patent Application Publication No. 2009-10926 (Patent Document 2).
However, the art disclosed in Non-Patent Document 1 aims to improve the electromechanical coupling coefficient of the aluminum nitride film, and thus fails to obtain an electromechanical coupling coefficient higher than that obtained from a material characteristic of the aluminum nitride film. In addition, the art disclosed in Patent Document 1 aims to improve the electromechanical coupling coefficient by increasing a bond concentration of a grain boundary between c-axis oriented aluminum nitride particles, and thus fails to obtain an electromechanical coupling coefficient higher than that obtained from a material characteristic of the aluminum nitride film.
The acoustic wave device grows in size and thus increases cost as a resonance frequency decreases.